1. Field of the Invention
The present invention relates to radio frequency identification (RFID) readers and transponders, and more particularly, to an RFID reader that can determine the direction of movement of a transponder relative to the reader in a modulated backscatter communication system.
2. Description of Related Art
In the automatic identification industry, the use of RFID transponders has grown in prominence as a way to track data regarding an object to which the RFID transponder is affixed. An RFID transponder generally includes a semiconductor memory in which digital information may be stored. A known technique for communicating with RFID transponders is referred to as xe2x80x9cbackscatter modulation,xe2x80x9d whereby the RFID transponders transmit stored data by modulating their antenna matching impedance to reflect varying amounts of an electromagnetic field generated by the RFID reader. An advantage of this communication technique is that the RFID transponders can operate independently of the frequency of the energizing electromagnetic field, and as a result, the reader may operate at multiple frequencies so as to avoid radio frequency (RF) interference, such as using frequency hopping spread spectrum modulation techniques. The RFID transponders may extract their power from the energizing electromagnetic field, thereby eliminating the need for a separate power source.
In many applications, it would be desirable for the RFID reader to derive location and direction information from the RFID transponder in addition to the stored data.
Typically, the determination of relative location between the RFID transponder and reader has been controlled by a combination of system sensitivity and antenna patterns. For example, an antenna of appropriate gain pattern can be used to provide coverage of a single lane of traffic so that an interrogator can establish communications with RFID transponders on vehicles in that lane and no other. This and similar techniques have been successfully used in systems where at most one object carrying a transponder can be physically located in the coverage area of the reader antenna. An ambiguity arises, however, when several transponders are located in the coverage area of the reader antenna. For such situations, it is necessary to physically determine which transponder has established communications with a reader, or alternatively, what the location of the transponder is with reference to the location and orientation of the reader.
There are many known techniques for determining the direction from a single location to a transponder using an array of antennas and measuring signal strength or arrival direction of the wave transmitted by the transponder. Hane (U.S. Pat. No. 4,728,955) describes one such technique to locate a modulated backscatter transponder using an antenna array. The transponder produces a single sideband suppressed carrier modulated backscatter signal that contains modulation of a subcarrier. The direction of arrival is determined by measuring the phase of the signal in each of several antennas and calculating the direction of arrival based on the measured phases. A significant drawback of this technique is that it is complex, cumbersome, and relies on carefully maintaining linearity in amplifiers and detectors. Moreover, the Hane technique includes measuring phase angles and using a computer to calculate the direction to the transponder, and is thus ill suited to receivers using limiting amplifiers, such as those shown by Koelle et.al. (U.S. Pat. No. 4,739,328).
Limiting amplifiers eliminate detailed phase information that would be necessary to accurately determine location using the Hane technique. The output of a limiting amplifier only provides information of whether the signal is from 0xc2x190xc2x0 from the reference phase (i.e., in phase) or is from 180xc2x190xc2x0 (i.e., out of phase) from the reference phase. All detailed phase information is therefore lost in a limiting amplifier.
More specifically, the Hane technique is ill suited for modulated backscatter systems that use direct modulation of the microwave carrier as opposed to the single sideband technique. That is because the output of the mixers in Hane will experience xe2x80x9cquadrature nullsxe2x80x9d with transponder position for transponders that do not produce a single sideband suppressed carrier signal. Koelle et. al. eliminate the xe2x80x9cquadrature nullxe2x80x9d effect by using a multi-channel receiver and limiting amplifiers in the reader to communicate with the transponders. Thus, the Hane technique would not provide direction information for a transponder of the type disclosed by Koelle et al. even if the mixers of Hane are replaced with a multi-channel homodyne receiver of Koelle et al. in view of the use of limiting amplifiers.
An alternative direction finding system was disclosed by Koelle et. al. (U.S. Pat. No. 5,510,795). According to Koelle et. al., the direction finding system measures whether a transponder is moving toward or away from the reader. If the transponder is moving past a reader, the direction finding system will provide an indication of when the movement of the transponder is zero in the direction of the reader. Unless the path of the transponder is restricted (e.g., mounted on an object on rails), the system cannot be used to determine the direction to the transponder. Likewise, the system cannot be used to determine the direction to the transponder if the reader antenna is rotated in a searching mode since the distance between a transponder and the reader does not change in that case.
It is also possible to determine the location of a transponder using a bi-static homodyne radio system where the transmit antennas and receive antennas are separated by a considerable distance as compared to the microwave wavelength. Such a geometry is described in R. J. King, Microwave Homodyne Systems, pp. 206-216 (1978). Communication with transponders takes place where the gain patterns of the transmit and receive antenna of the reader system intersect. That intersection defines the area in space where communications with the transponder are possible. Such a system is not useful for a compact, handheld reader nor for determining the location of a particular transponder if several transponders are located in the communication zone.
Other direction finding techniques are used for determining the direction to a conventional radar target. A method known as xe2x80x9csimultaneous lobingxe2x80x9d or xe2x80x9cmonopulsexe2x80x9d is described in M. I. Skolnik, Introduction to Radar Systems, pp. 175-184 (McGraw-Hill 1962). According to this method, collocated or closely spaced antennas forming sum and difference beams use phase and/or amplitude detectors to determine precisely when a radar beam is swept across a remote target. A drawback of this method is that it cannot be used to determine the direction to a backscatter transponder when it is in the vicinity of other scattering objects which produce signals stronger than those of the transponder. Additionally, typical antenna patterns of reader antennas are of relatively low gain (e.g., 6 to 15 dBi) because of the necessity of providing the required coverage area. If low gain antenna elements are used, the offset feed technique to produce two beams to form sum and difference beams results with a difference beam having very small gain, which is not useable for RFID applications. Thus, the normal direction finding techniques used in conventional radar systems are not applicable to modulated backscatter systems.
Accordingly, it would be desirable to determine the direction from a reader to a transponder in a backscatter communications system where the amplification of signals is performed using limiting amplifiers.
The present invention discloses methods and systems to determine the direction to a transponder in a modulated backscatter communication system. More particularly, such systems and methods make use of at least two antennas to receive a scattered or modulated signal from a transponder, such as a RF ID backscatter tag. An additional antenna is used as a reference from which the direction of the transponder is determined and is also used to transmit the signal to the transponder which then becomes modulated or scattered.
The signal received on the two antennas is used to form a difference signal. Since receiving the signal through the two antennas could have the effect of producing two different versions (i.e., different phases) of the signal, subtracting the outputs of the antennas will produce a difference signal that may not have a value of zero. That difference signal is delayed by 90 degrees. The polarity of the delayed signal is compared with the polarity of the scattered or modulated signal as received by the reference antenna to determine the direction of the transponder in relation to the reference antenna.
A first object of the present invention is to provide a simple yet accurate scheme for determining the direction of a transponder in relation to a reference antenna. Another object of the present invention is to provide a simple circuit for implementing the methods of the present invention. Still another object of the present invention is to enable the use of the disclosed systems and methods with backscatter tag receivers that use limiting amplifiers.